1. Field of the Invention
The invention relates to a vehicular differential apparatus that distributes a driving force of a vehicle between paired output shafts with a differential between the output shafts permitted.
2. Description of the Related Art
In the related art, between a right drive shaft and a left drive shaft of a vehicle, a differential apparatus is provided which includes: a differential case to which a driving force generated by a driving source such as an engine is input; a plurality of pinion gears that is rotationally held in the differential case and that is revolved by rotation of the differential case; and a pair of side gears housed coaxially in the differential case so as to be rotatable relative to the differential case, and in which the input driving force is distributed between paired output shafts with differential between the output shafts permitted. Some such differential apparatuses include a differential limiting function to limit differential rotation between the output shafts by a frictional force to enhance straight-ahead traveling capability and thus traveling safety of the vehicle and to enable suppression of idle running (slip) of wheels on low μ roads such as pressed snow roads and muddy roads (see, for example, Japanese Patent Application Publication No. 2009-174577 (JP 2009-174577 A).
In the differential apparatus described in JP 2009-174577 A, a pair of sun gears functioning as the side gears is configured such that each sun gear has two members, namely an inner portion coupled to the corresponding output shaft and an outer portion engaged with a pinion gear (planetary gear), and a thrust force generating mechanism that is helical teeth is provided between the inner portion and the outer portion. In the thrust force generating mechanism, respective thrust forces in opposite directions are generated in the inner portion and the outer portion to increase frictional resistance generated at end surfaces of each of the inner and outer portions in an axial direction. The frictional resistance limits the differential between the output shafts.
A center washer is disposed between the outer portions of the sun gears, and a center washer different from the center washer between the outer portions is disposed between the inner portions of the sun gears.
The differential apparatus described in JP 2009-174577 A has two center washers, which results in an increased number of components and increased assembly man-hours. Thus, a single center washer may be used instead of the two center washers and disposed between the inner portions and between the outer portions of the sun gears. However, in this case, when the inner portion and the outer portion of each sun gear are designed to be identical in axial length as described in JP 2009-174577 A, the differential limiting force may be varied by a slight difference in axial length resulting from a machining error.
In other words, when the outer portion of each sun gear is slightly longer than the inner portion of the sun gear, if the center washer is moved toward one side in the axial direction by a thrust force, the outer portion of the sun gear on this side is subjected to a stronger thrust force than the inner portion of the sun gear. When the inner portion is slightly longer than the outer portion, the inner portion is subjected to a stronger thrust force than the outer portion. Due to a difference in diameter between the inner portion and the outer portion, resultant rotational resistance, that is, the resultant differential limiting force, may vary between the inner and outer portions even when the inner and outer portions are subjected to a thrust force of the same magnitude. Thus, the differential limiting force is likely to vary (differences among individual products).